Cellulosic material, such as paper, wood, rayon and cotton are highly flammable substances and when ignited, the fire spreads rapidly. Many methods for retarding the spread of such fires are known, but for many commercial uses, such methods are often too costly or ineffective or render the physical characteristics of the flame retarding product undesirable.
Cellulosic materials used throughout the industry and the home are closely associated with humans. Consequently, methods of chemically treating them to resist burning must assure a safe living environment and preserve their aesthetic qualities, particularly when the product is wood. The use of volatile or harmful solvents should be minimized during the process for treating the product with a chemical flame retarding agent, especially when a residue of such a solvent would exude from the processed material. The chemical treatment also should not produce or leach toxic or ugly deposits onto the surface of the material and must also maintain the aesthetic qualities of the material through the broad range of environmental conditions it may be subjected to.
Chemical methods for flame retarding treatment of ligno cellulosic material, such as plywood panels, furniture wood, cardboard, paper and the like, include the application of salts of phosphoric acid, boric acid, sulfamic acid and the like as well as various organic compounds in most instances in organic solvent systems. These methods are economically attractive but cause crystallization of salts on the surfaces of the treated wooden product after drying, and cause the migration of undesirable chemicals to the surfaces. In addition to these difficulties, the solvents used are volatile during the application of the flame retardants and exude from the finished product.
Ammonium phosphates are among the more effective inorganic fire retardants which are commonly used to treat cellulosic material. Such phosphates are typically present as diammonium phosphate, monoammonium phosphate, or simple or complex mixtures of such phosphates. Particularly suitable fire retardants of this variety are prepared by reacting aqueous phosphoric acid with an alkylene oxide, such as ethylene oxide, propylene oxide or butylene oxide. See U.S. Pat. No. 3,900,327, exemplifies such fire retardants formed by reacting 0.5 to 1.5 parts of ethylene oxide by weight of orthophosphoric acid. An improved fire retardant of this variety is disclosed in U.S. Pat. No. 4,383,858 wherein an alkylene oxide of 2 to 4 carbon atoms is reacted with aqueous phosphoric acid, with the weight ratio of oxide to acid being in the range of from about 0.01:1 to about 0.25:1.
Commercial processes for treating wood panels involve the use of automated high speed equipment which subject the panels to consecutive treatment processes. The fire retardant material is usually applied to the panels in one of these treatment processes. During the application process, the panel surface is covered with an appropriate quantity of fire retardant in liquid form. Application methods generally include spraying, immersion, flooding, bath purging, drip flushing, batch and continuous processing through tanks, dips or through sprays, by wick or absorptive fiber treatment or by other similar known methods. In a typical process for treating wood panels, 4 feet.times.8 feet sheets of plywood on a high speed assembly line are preheated to a temperature of about 40.degree. C. to 150.degree. C. and the top surfaces are over-sprayed or coated with fire retardant using a wick or absorptive fiber method or a spray stream, mist, drip or surface sheet dip method. Excess composition is removed from the surfaces after intimate contact for at least about one to three seconds. The plywood is then subjected to a drying cycle which dries the panels in a hot over in a few seconds. The dried panels are then sanded in a continuous operation.
During the drying cycle, the surface temperature of the panels can reach 220.degree. F. to 230.degree. F. or more. Temperatures of this magnitude have the undesirable effect of causing the fire retardant material to exude from the surface of the panels, forming salt deposits which in turn form blisters as water is vaporized from the interior of the panels. Not only does this result in the loss of significant amounts of fire retardant, but the surface salts cause clogging of the sandpaper which must then be periodically cleaned requiring stoppage of the assembly line. The alternative of wet-stacking the panels to dry overnight is not a cost effective procedure.
It is therefore a principal object of the present invention to provide an improved ammonium phosphate-containing fire retardant composition which is less prone to exudation from the treated wood panels during continuous treatment processes.